The present invention generally relates to devices which affect the transmission of optical fiber signals. More particularly, the present invention relates to optical waveguide attenuating devices.
In optical fiber communication systems which transmit optical signals over long distances, it is generally desirable to minimize light losses due to absorption and scattering in the optical fibers. In many instances, however, it is necessary to employ optical attenuator devices to reduce the amount of power present in the optical signal.
For example, communication system receivers optimally function within a certain range of an optical input level, and it is therefore necessary to adjust the input level to the desired range. The path attenuation in an optical communication system is a function of fiber length and the fiber attenuation coefficient. Thus, the path attenuation can be adjusted by increasing the fiber length, but this is not practical using low loss fibers. Accordingly, high loss optical fiber attenuators are used to increase the path attenuation where it is impractical to use long lengths of low loss fibers. Attenuators also are utilized to equalize optical signals from different sources, or to simulate the presence of a long low loss line when calibrating an optical component or network.
Fiber attenuators are also used at terminations for the ends of unused optical fibers of devices such as wavelength division multiplexers and star couplers to eliminate unwanted back reflections. The use of fiber attenuators as terminators for unused fibers in star couplers is described in U.S. Pat. No. 5,572,618, which is relied upon and incorporated by reference.
Optical fiber attenuators typically comprise an optical fiber having a core and a cladding, the core including a dopant material which increases the attenuation of the optical fiber. The dopant can be introduced by solution doping transition of rare earth elements into the fiber's core, and the attenuation of the fiber is directly proportional to the dopant concentration and the fiber length. See, for example, U.S. Pat. No. 5,633,974. Solution doping has several disadvantages, however, chiefly that solution doping involves an additional processing step which must be performed after fabrication of the optical fiber.
Another fiber attenuator described in U.S. Pat. No. 4,881,793 involves vapor deposition doping the core of an optical fiber with a variable valency element such as Ti, V, Cr, or Fe, which is partially present in a lower valency state. The presence of the lower valency state is controlled by the amount of oxygen used during the deposition process or by consolidating the preforms in a reducing atmosphere. The attenuations achieved with the fiber described in U.S. Pat. No. 4,881,793 are reported as only reaching about 25 dB/m. In certain applications, such as terminators, it is desirable to provide an attenuator having an attenuation in excess of 100 dB/m.
Accordingly, it would be desirable to provide a method for fabricating an optical attenuating device capable of providing a broad range of attenuations.